This invention relates to a semiconductive silicone rubber composition which cures into silicone rubber having a stable resistivity in the semiconductive region and is suitable as a roll material for use as rolls in business machines. It also relates to a silicone rubber roll obtained therefrom.
A roll material is one application of rubber material in the business machine area. A wide variety of rubber materials including urethane elastomers, ethylene-propylene rubber and natural rubber as well as silicone rubber are currently used for such purposes. Composites of these materials are also known. These rubber materials have been used in a variety of applications as an electrically insulating material or an electroconductive material having a conductive agent blended therein for the purposes of reducing static-electrical charges and establishing electrical conduction paths. Recently, semiconductive materials having a resistivity in the medium range of 1xc3x97103 to 1xc3x971012 xcexa9-cm find use as a roll material in copiers.
Since a semiconductive roll material controls current flow therethrough by its own resistance in most service environments, the stability of its resistivity is very important in order to acquire a transfer capability. A voltage of 100 V to 10 kV is often applied across the roll material in order to stabilize the current flow therethrough and to provide effective electrical charges onto the organic photoconductor (OPC).
However, most semiconductive roll materials suffer from resistance instability because they use carbonaceous conductive agents. Under the high-voltage service environment, the stability of carbon as the conductor encounters a certain limit. During long-term service, the resistance deviates from the preset value due to failure of the carbon-to-carbon structure and volatilization of carbon. This problem becomes more outstanding as the applied voltage becomes higher and as more current is conducted. It is known that where conventional conductive carbon black is used, the current-to-voltage relationship does not follow Ohm""s law and becomes nonlinear because of the tunnel effect of carbon structure (see L. K. H. van Beek and B. I. C. F. van Pul, J. Appl. Polymer Sci., 6, 651 (1962)). The nonlinearity between current and voltage is irrational as the properties of OPC-related rolls such as developing and transfer rolls whose resistance must be precisely controlled within the semiconductive region. This problem can be solved by adding phosphates as described in JP-A 6-192486 or potassium titanate whiskers as described in JP-A 63-86205 and JP-A 8-120176, but to an unsatisfactory extent. Also JP-A 10-176111 discloses a semiconductive silicone rubber composition having conductive titanium oxide whiskers blended therein, whose resistance is not fully stable under high voltages.
An object of the invention is to provide a semiconductive silicone rubber composition which cures into a silicone rubber having a stabilized resistance even under high voltages and exhibiting a nearly linear current-to-voltage relationship under high voltages and thus suitable as roll material. Another object of the invention is to provide a silicone rubber roll having such advantages.
It has been found that by blending acicular conductive titanium oxide and an antistatic agent having a metal ion-containing moiety in the molecule in an organopolysiloxane of the formula (1) shown below, there is obtained a silicone rubber composition which is readily workable and curable with an organic peroxide or an organohydrogenpolysiloxane/platinum group catalyst or a mixture of an organic peroxide and an organohydrogenpolysiloxane/platinum group catalyst. The composition cures into silicone rubber having improved compression set and stable conductivity or resistivity in the semiconductive region, especially in the range of 1xc3x97103 to 1xc3x971012 xcexa9-cm. Since the deterioration of the conductive agent under high voltages is prohibited, the molded or cured product experiences a minimal change of resistance and withstands long-term service. The composition is thus suitable as a semiconductive roll material for use as charging rolls, transfer rolls and developing rolls in business machines such as copiers, facsimile machines and printers.
The invention provides a semiconductive silicone rubber composition comprising
(A) 100 parts by weight of an organopolysiloxane having at least two aliphatic unsaturated groups in the molecule represented by the following average compositional formula (1):
R1nSiO(4xe2x88x92n)/2xe2x80x83xe2x80x83(1) 
xe2x80x83wherein R1, which may be the same or different, is a substituted or unsubstituted monovalent hydrocarbon group and n is a positive number of 1.98 to 2.02,
(B) 1 to 100 parts by weight of acicular conductive titanium oxide,
(C) 0.5 to 50 parts by weight of an antistatic agent having a metal ion-containing moiety in the molecule, and
(D) a sufficient amount to cure component (A) of a curing agent.
In one preferred embodiment, the composition further includes a blowing agent. Preferably the curing agent is a combination of an organohydrogenpolysiloxane with a platinum group catalyst or an organic peroxide or both.
The invention also provides a semiconductive silicone rubber roll comprising a semiconductive silicone rubber layer obtained by curing the composition.